The present invention relates to a measuring device employing laser interferometry, and which provides for determining the displacement of a reflector within a given working cone, by virtue of a reflector tracking system designed to eliminate any error in measurement.
Interferometry, particularly laser interferometry, consists in determining the interference fringes produced by the increase in length of a light beam in relation to a reference beam; both beams being generated by the same source, and being divided by a separator (plate, semireflecting mirror, polarizing prism, Nicol prism . . . ), after which, they are reflected by a fixed and mobile reflector respectively on to the said separator where they are re-united. When the mobile reflector is moved, the reunited reflected beam presents interference, which is counted by means of a cell; each fringe corresponding to a half wavelength of light in air (approximately 0.3 .mu.m on standard lasers employed in applications of this sort).
One of the major drawbacks of equipment of this type consists in obtaining the reflected beam from the mobile reflector. Reflectors are known to consist of concave right-angle mirrors, which send back a reflected beam parallel with the incident beam and symmetrical in relation to their edges. For measuring or controlling displacement of the reflector, the displacement direction must therefore be known, and the equipment set up accordingly for ensuring interception of the beams directed on to the mobile reflector, and the formation of a useful reflected beam.
In certain applications, for measuring the distance between two points on a structure, the target (mobile right-angle reflector) is mounted on a support which can be moved from one point to another on the test structure. As the said support is moved manually, the mobile reflector is mounted on a device which moves it, in relation to the support, in a plane perpendicular to the said manual displacement, so as to maintain the return beam from the mobile reflector within a given area enabling it to be reunited with the return beam from the fixed reflector. The said device for displacing the mobile reflector is connected to a cell for ensuring maximum illumination. In addition to being limited to special measuring applications, the said device also involves a certain amount of inaccuracy, due to inevitable, poorly controlled slack between the mobile reflector and support.
A basic interferometric cell is therefore required for tracking the path of the mobile reflector within given limits. A number of cells assigned to one or more mobile reflectors, mounted on freely displaceable measuring supports of lightweight, low-cost design, therefore provide for achieving at least the same measuring accuracy as much larger climatized machines currently available on the market.